Institute of Infectious Disease and Molecular Medicine, and Division of Medical Biochemistry, University of Cape Town, Cape Town, South Africa
Correspondence: Edward D Sturrock, Division of Medical Biochemistry, Institute of Infectious Diseases and Molecular Medicine, Faculty of Health Sciences, University of Cape Town, Anzio Road, Observatory 7925, Cape Town, South Africa. Email: firstname.lastname@example.org
The renin–angiotensin system (RAS) is central to regulation of blood pressure and electrolyte homeostasis.
Angiotensin-converting enzyme (ACE), a key protease in the RAS, has a range of substrates, including N-acetyl-Ser-Asp-Lys-Pro (Ac-SDKP). The peptide Ac-SDKP is cleared almost exclusively by ACE, and specifically by the N-domain active site of this enzyme.
N-Acetyl-Ser-Asp-Lys-Pro is a negative regulator of haematopoietic stem cell differentiation and is a potent antifibrotic agent. In this review, the physiological actions of Ac-SDKP are presented, together with the potential clinical usefulness of raising Ac-SDKP levels. This emphasizes the possible opportunity of N-domain-selective ACE inhibitors or ACE-resistant Ac-SDKP analogues for the treatment of fibrosis.
Introduction: Angiotensin-Converting Enzyme and the Renin–Angiotensin System
The renin–angiotensin system (RAS) is a central system in the regulation of blood pressure and electrolyte homeostasis. This system involves the conversion of the hepatic precursor protein angiotensinogen to the inactive decapeptide angiotensin (Ang) I by the highly substrate-specific aspartyl protease renin. Angiotensin-converting enzyme (ACE) then cleaves AngI to give the active hormone AngII, which mediates its effect on blood pressure via the angiotensin receptors (AT1 and AT2).[1, 2] Inhibition of ACE and antagonism of the AT1 receptor are well-established therapeutic interventions for hypertension and cardiovascular disease, but investigation of the AT2 receptor as a pharmacological target has lagged behind. Stimulation of the AT2 receptor and activation of its tissue-protective features has recently proven to have therapeutic effects in animal models of cardiovascular, renal, neurological and inflammatory disease. In addition, ACE interacts with the kinin–kallikrein system by inactivating the vasodilator bradykinin, which is produced by proteolytic cleavage of its kininogen precursor.
The classical view of the RAS as a circulating endocrine system where angiotensingogen is converted stepwise to the vasoconstrictor AngII has changed dramatically in recent years. The discovery of the carboxypeptidase-like enzyme ACE2, which hydrolyses AngII to Ang-(1–7), provides a counterbalance to the vasoconstrictive and/or proliferative RAS axis, with Ang-(1–7) functioning as a physiological antagonist to AngII through the Mas receptor.[5-7] Moreover, AngII is also converted to AngIII by aminopeptidase A and AngIII is converted to AngIV by cleavage of Asp and Arg,respectively. Angiotensin IV binds to the AT4 receptor, identified as the insulin-regulated aminopeptidase, which is widely expressed in the kidney, brain, heart and vessels. Angiotensin IV has been shown to play a role in learning, memory and cognitive improvement, and AngIV analogues are a potential therapeutic intervention for Alzheimer's disease-associated memory loss (Fig. 1).
“classical view of the RAS has changed dramatically”
Although ACE occupies a central position in the RAS cascade, it is also involved in a number of functions outside the RAS, such as reproduction, haematopoiesis, the immune response and kidney development and function (for a review, see Shen et al.). This functional diversity of the enzyme is largely due to it wide substrate specificity, which is facilitated by two catalytic sites contained in the homologous tandem domains, termed N- and C-domains. Animal studies using transgenic mice that express ACE with inactivated N- or C-catalytic sites have provided valuable insights regarding the in vivo functions and synergy of these structurally similar but non-identical substrate binding sites.[12-14] It is the purpose of the present minireview to provide a brief overview of the physiological relevance and roles of N-acetyl-Ser-Asp-Lys-Pro (Ac-SDKP), an ACE substrate outside the RAS, as well as to present the opportunities for the design of ACE inhibitors that could mediate the beneficial effects of Ac-SDKP build up.
Angiotensin-Converting Enzyme and Ac-SDKP: Expanding Biological Roles
Approximately two decades ago, ACE was discovered to be specific in the degradation of Ac-SDKP in plasma, with coincubation with 1 μmol/L of an ACE inhibitor almost completely preventing Ac-SDKP hydrolysis. In both mouse and human studies, plasma levels of Ac-SDKP are significantly elevated following treatment with an ACE inhibitor.[16, 17] Intriguingly, this peptide hydrolysis seemed to be mediated primarily by the N-domain active site of ACE. In addition, mice that have been treated with N-domain selective ACE inhibitors or have genetically inactivated N-domain active sites have elevated Ac-SDKP similar to complete ACE inhibition controls.[12, 19]
The peptide Ac-SDKP arrests the proliferation of bovine haematopoietic pluripotent stem cells. It arises from the cleavage of thymosin β4 by prolyl oligopeptidase (POP; Fig. 1).[21, 22] More recent work has also underlined important tissue antifibrotic and pro-angiogenic roles of Ac-SDKP.
Physiological Roles of Ac-SDKP
The biological activity of Ac-SDKP was first discovered in light of its ability to limit the proliferation of blood stem cells, which was found to be endogenous in mice, secreted by bone marrow in long-term culture and had wide tissue distribution.[25, 26] It has been shown that Ac-SDKP inhibits G1–S phase transition in the cell cycle and thus keeps progenitor cells in the quiescent state. Importantly, cotreatment with Ac-SDKP increased survival in mice given a lethal dose of cyclophosphamide, enabling higher doses of chemotherapeutic agents to decrease tumour size without causing death.[28, 29] In addition, Ac-SDKP was shown to inhibit proliferation of normal progenitor cells, but not already committed leukaemic cells,[30, 31] raising expectations that elevated Ac-SDKP levels could serve as a useful adjunct to lower chemotherapeutic-associated morbidity by keeping progenitor cells in the quiescent state during treatment.
“Ac-SDKP is elevated following treatment with ACE inhibitor”
Given the in vivo Ac-SDKP clearance by ACE, it was anticipated that ACE inhibitors could yield similar antiproliferative results, and initial in vitro and in vivo results yielded promising findings. In clinical studies, patients undergoing monochemotherapy (with either cytarabine or isofamide) were coadministered Ac-SDKP and showed protection of peripheral blood cells and lowered toxicity. Little development has taken place in the literature in the past 12 years regarding the use of Ac-SDKP in chemotherapy treatment, suggesting that translation into the human setting has not been as effective as anticipated. No clinical trials have been published assessing ACE inhibitors in haemoprotection.
Studies over the past decade, particularly by Carretero et al., have resulted in an appreciation of another activity of Ac-SDKP that could have clinical benefit (summarized in Table 1). In an initial cell culture model system, Ac-SDKP was able to inhibit DNA synthesis and endothelin-1-induced collagen synthesis in rat cardiac fibroblasts, confirming that this peptide can mediate its effects on cell types other than the haematopoietic system. The same effect was shown in vivo with aldosterone salt-induced hypertensive rats, whereby coadministration of aldosterone salt with Ac-SDKP significantly lowered collagen deposition and cell proliferation in the left ventricle and renal cortex without affecting blood pressure. Similar effects have been noted for a number of tissues in mouse models of hypertension and myocardial infarction, including interstitial and perivascular tissue of the heart,[35-39] aortic tissue and renal tissue.[41-43] Inhibition of Ac-SDKP production by POP inhibitors revealed that the regulation and minimization of fibrosis is possibly the major role of this peptide at endogenous levels. Although all the aforementioned cases occur in models of hypertension or infarction, the role of Ac-SDKP in fibrosis is by no means limited to such scenarios. This peptide also has the ability to lower collagen deposition in diabetic rats, to attenuate liver fibrosis and inflammation in rats treated with carbon tetrachloride, to reduce bile duct ligation-induced liver fibrosis, to decrease renal fibrosis in lupus nephritis, to reduce both lung silicotic fibrosis[49, 50] and drug-induced lung fibrosis and to alter myocardial fibrosis in experimental rat autoimmune myocarditis. Furthermore, decreased Ac-SDKP levels have been detected in the pericardial fluid of tuberculosis patients with constrictive pericarditis compared with uninfected controls.
Table 1. Summary of antifibrotic effects mediated by N-acetyl-Ser-Asp-Lys-Pro in various rat or mouse models
Identification of the predominant antifibrotic role of Ac-SDKP also suggested an additional mechanism of action for ACE inhibitors, whereby the reduction of fibrosis is not due to the reduction of profibrotic AngII levels alone, but also by the elevation of Ac-SDKP. The exact nature of these contributions has been carefully elucidated by Carretero et al. in different hypertensive models.[38, 39] Both ACE inhibitor treatment and Ac-SDKP infusion reduced cardiac fibrosis. However, when cotreated with an antibody specific for neutralizing Ac-SDKP, this resulted in restoration of excessive collagen deposition comparable to untreated controls. This suggests that, at least under the conditions of these models, the decrease in fibrosis produced by ACE inhibition is due, in large part, to Ac-SDKP build up and not due to AngII inhibition, suggesting that inhibition of the N-domain of ACE may have a dominant role in this therapeutic effect.
“minimization of fibrosis the major role of Ac-SDKP”
Currently understood molecular mechanism of action
More recent studies have started to delineate the molecular mechanism of the antifibrotic effect of Ac-SDKP. This peptide slows differentiation of bone marrow-derived stem cells and thus limits proliferation, migration, activation and cytokine release of macrophages, presenting a novel anti-inflammatory mechanism.[36, 37, 39, 53] Furthermore, Ac-SDKP limits the infiltration of leucocytes into cardiac tissue by affecting extracellular matrix composition, thus decreasing cytokine presence in the affected tissue. In addition, Ac-SDKP prevents maturation of human cardiac fibroblasts and subsequent collagen production when fibroblasts are exposed to transforming growth factor (TGF)-β, a fibrosis-promoting cytokine.
“Ac-SDKP downregulates TFGβ/Smad and ERK pathways”
The effects of TGF-β signalling are blunted by Ac-SDKP via downregulation of the TGF-β/small mothers against decaplentaplegic (Smad) and extracellular signal-regulated kinase (ERK) 1/2 pathways.[54, 55] Effects mediated by galectin-3, a TGF-β-releasing, macrophage-recruiting and cardiac dysfunction molecule, are known to be blunted by this mechanism. However, the exact mechanism of Smad pathway regulation remains unknown and does not seem to involve increasing intracellular concentrations of inhibitory Smad7. Recently it was shown that endothelin-1-downregulated sarcoma homology 2-containing protein tyrosine phosphatase activity can be counteracted by Ac-SDKP in a dose-dependent manner, suggesting that Ac-SDKP can protect the activity of phosphatases and therefore the removal of pertinent activated signalling molecules produced by hormonal signals. Interestingly, Ac-SDKP has been shown to bind specifically to an as yet uncharacterized receptor in rat cardiac fibroblasts. It is therefore reasonable to speculate that this peptide could mediate its effects through independent receptor binding, and this is an area of current investigation.
Although Ac-SDKP has no effect on blood pressure, recent data suggest that it may affect tissue expression of RAS proteins. In a silicotic model of lung fibrosis, Ac-SDKP infusion decreased AT1 receptor lung expression. Moreover, zymography studies indicate that Ac-SDKP can reduce matrix metalloprotease (MMP) activity in myocardial tissue of an autoimmune myocarditis rat model, which could have implications in leucocyte infiltration. This opens up interesting avenues for future research, whereby ‘separate’ systems of the RAS, MMPs and Ac-SDKP participate in some sort of regulatory feedback and provide a possible link to regulating the profibrotic effects of the AngII-activated AT1 receptor and leucocyte infiltration.
Possible Clinical Applications and Benefits
There are several exciting potential clinical applications and benefits, but it is important to stress that all of these still require proof-of-concept in the clinic. Thus, although mouse and rat models may be informative as to the therapeutic potential of Ac-SDKP, these findings need to be extended into the human setting to assess possible future use. It is anticipated, given the potential of fibrotic disease treatment presented below, that such assessments in the clinic are indeed appropriate. As noted above, many conditions could be effectively treated or controlled by increasing Ac-SDKP levels without affecting blood pressure (Table 1). There is a significant unmet medical need in the treatment of various forms of pathological fibrosis, which can affect most major organ systems.[58, 59] Fibrosis can result from acute or chronic injury or from poorly understood autoinflammatory and autoimmune diseases, including interstitial fibrosis of the lung and systemic sclerosis. These processes often involve TGF-β, a pathway that is modulated by Ac-SDKP. Bleomycin, a potent antineoplastic agent, is often discontinued due to lung fibrosis. However, mice lacking the N-domain active site of ACE, and therefore have increased Ac-SDKP levels, display significantly lower pulmonary collagen deposition and an increased survival with a lethal drug dose compared with their wild-type littermates. Furthermore, a model of lung silicosis showed that treatment after agitator exposure still reduced fibrosis. These are exciting preliminary results that require further validation before their evaluation in pilot clinical studies. Although N-domain ACE inhibitors have the theoretical advantage of raising Ac-SDKP levels without affecting blood pressure, there may be clinical situations in which lowering blood pressure is not significant, which would enable proof-of-principle testing with current ACE inhibitors.
Furthermore, although there are only preliminary results, it has been proposed that Ac-SDKP could be used as a marker for tuberculous pericarditis and certain haematological malignancies.[52, 61] In addition, given the tissue injury and long duration of antituberculous treatment, Ac-SDKP levels could serve as a useful biomarker of organ fibrosis and treatment response. Another possibility that has been discussed recently is the use of Ac-SDKP in the reduction of soft tissue injury due to artificial polymeric implants.
Angiotensin-converting enzyme is a central feature of the RAS and, although not classed in the same system, the ACE substrate Ac-SDKP is an important regulator of physiologically relevant events. Current data suggest that Ac-SDKP has promise as a potent antifibrotic agent. Elevation of Ac-SDKP through infusion could be a viable approach in disease treatment. However, given the specific clearance of this peptide by ACE, specific ACE inhibition could be particularly useful in helping to treat the conditions described above. Selective inhibition of the N-domain could mediate these potential beneficial effects of Ac-SDKP build up without affecting blood pressure. The usefulness of current clinical ACE inhibitors, together with the development of Ac-SDKP analogues resistant to ACE cleavage and the design of clinically relevant N-domain-selective ACE inhibitors remain the focus of current research. Strategies for the treatment and management of fibrosis are important because treatment options for fibrotic conditions are currently limited.
“inhibition of N-domain could mediate beneficial effects of Ac-SDKP”